Slab Foundation Question ??

I recently purchased a home that was constructed on a slab foundation. When you walk into the house there is a small entry way that is approximately 12" lower then the rest of the house, there are 2 steps you need to walk up to enter the core area of the house. I just assumed that there were sleepers installed but when I pulled up the carpet I noticed it was all solid concrete.

My question is , could this be a solid slab that is up to 16" deep in the core area of the house or is there some other explanation for this?

It's highly unlikely that the slab is that thick. It's not that hard to excavate the plot to have a depression like that for the entryway. All it takes is some creative form making and rebar bends to hold things together.